Brushless d-c motors are widely known and readily made. A motor of the general type to which the present invention relates is described in U.S. Pat. No. 3,873,897, assigned to the assignee of the present application. Such a motor has a permanent magnet rotor which reacts with electric current applied in pulses, selectively, to either one of two winding groups--in which the winding groups may well have only a single winding--so that current flow through the respective winding groups interacts with a selected pole of the rotor. The current is alternatingly supplied, in pulses, unidirectionally, to the respective windings under control of switching circuits, for example control transistors.
The motor of U.S. Pat. No. 3,873,897 is efficient in utilization of the magnetic material present therein and requires only a simple and hence inexpensive electronic control circuit. Only a single electromagnetic control element, typically a Hall generator is used, located within the field of the rotor magnet and controlled thereby. This Hall generator, thus, is positioned in the air gap of the motor so that no special space is required therefor. The drive power for the motor is provided by current pulses selectively steered through the respective coils or groups of coils by the transistor switching circuit. The current pulses provide interrupted drive power; to render the output torque of the motor essentially uniform, an additional auxilliary torque, that is, a reluctance torque, is provided. This reluctance torque is matched to the overall, essentially uniform torque to be derived from the motor. Without such a reluctance torque, the output torque derived from the motor would occur in interrupted pulses, that is, have gaps therebetween. Such gaps in output torque are not desirable in many applications, and in some must be definitely prevented.
A brushless d-c motor which does not utilize a reluctance torque is described in U.S. Pat. No. 3,299,335. To provide for commutation control, an optical sensing system is provided. Actual operating experience has shown that optical sensing of position of a rotor in a motor is less reliable than galvanomagnetic sensing means, typically a Hall generator. If a galvanomagnetic rotor position sensing element were to be used in the motor of U.S. Pat. No. 3,299,335 to provide for more reliable commutation, then an additional permanent magnet rotor would be required, which would be undesirable due to the increase in motor size and consequent added cost.